[BUG] Add more validation checks for message API

[mingshl]

What is the bug?

In testing the message API,

• the message can be empty, it creates new messages with fields in null values.
• when sending fields that is not defined in the mapping, it also creates new messages with fields in null values. It doesn't throw exception.

We need to add validation and throw proper exceptions.

How can one reproduce the bug?

POST /_plugins/_ml/memory/
{
  "name": "Conversation for a RAG pipeline"
}

POST /_plugins/_ml/memory/-IyfjZMBQoVk1Lk93xvv/messages
{

}

GET /_plugins/_ml/memory/message/-YyljZMBQoVk1Lk99BtH

##returnning  new message -YyljZMBQoVk1Lk99BtH  with fields in null values.  
{
  "memory_id": "-IyfjZMBQoVk1Lk93xvv",
  "message_id": "-YyljZMBQoVk1Lk99BtH",
  "create_time": "2024-12-03T17:51:23.713738264Z",
  "input": null,
  "prompt_template": null,
  "response": null,
  "origin": null,
  "additional_info": {}
}

POST /_plugins/_ml/memory/-IyfjZMBQoVk1Lk93xvv/messages
{
   "message":{"role":"system","content":"Hi!"}
}

GET /_plugins/_ml/memory/message/giiijZMBjeltLmSpmHa5
##returnning message giiijZMBjeltLmSpmHa5  with fields in null values.  
{
  "memory_id": "-IyfjZMBQoVk1Lk93xvv",
  "message_id": "giiijZMBjeltLmSpmHa5",
  "create_time": "2024-12-03T17:47:43.667740682Z",
  "input": null,
  "prompt_template": null,
  "response": null,
  "origin": null,
  "additional_info": {}
}

What is the expected behavior? throw proper exception to guide users to send in valid messages.

What is your host/environment?

• OS: [e.g. iOS]
• Version [e.g. 22]
• Plugins

Do you have any screenshots? If applicable, add screenshots to help explain your problem.

Do you have any additional context? Add any other context about the problem.

[rithin-pullela-aws]

@dhrubo-os, can you please assign this issue to me.